In a general sense, this invention relates to synthetic resins. More specifically, it relates to flame-retardant polyphenylene ether compositions.
Polyphenylene ether (“PPE”) resins are commercially attractive materials because of their unique combination of physical, chemical, and electrical properties. The resins are usually characterized by a desirable combination of hydrolytic stability, high dimensional stability, toughness, heat resistance, and dielectric properties. They also exhibit high glass transition temperature values, typically in the range of about 150° C.–210° C., as well as good mechanical performance.
Polyphenylene ether-based resins are sometimes used for applications that require significant flame retardance. Examples include the construction and transportation industry. As one specific example, some of the PPE compositions can be used as insulating materials for electrical wire. Polyphenylene ether by itself is somewhat flame-retarding, and can be useful for certain end use applications. However, the combination of PPE with other polymers such as styrene resins can significantly decrease flame retardance, in the absence of flame-retardant additives.
A large number of flame-retardant additives and modifiers have been used in polyphenylene ether compositions. As one example, U.S. Pat. No. 4,808,647 (V. Abolins et al.) describes the use of a multi-component flame-retardant composition which includes an organic phosphate material. The compositions also include a brominated material, such as a tetrabromobisphenol-A oligomeric polycarbonate.
These types of compositions exhibit a number of desirable properties suitable for many applications. However, it is often desirable to minimize or eliminate the use of bromine or other halogen-containing additives in compositions designed for certain products. For example, the wire insulation compositions mentioned above frequently cannot contain halogen additives because they could form corrosive compounds when exposed to flame. The presence of the corrosive compounds may result in considerable damage to surrounding electrical equipment.
Flame-retardant compounds which are generally free of halogens are described in various references. For example, Patel (U.S. Pat. No. 6,576,700) describes the use of various classes of phosphorous-based flame-retardants in polyphenylene ether resins. Examples include elemental phosphorous, organic phosphonic acid, phosphonates, phosphinates, phosphinites, phosphine oxides, and phosphates. Of particular interest are a number of phosphates and diphosphates. Some of these are also described by Hellstern-Burnell et al, in U.S. Pat. No. 5,294,654. Examples include tetraphenyl bisphenol-A diphosphate; tetraphenyl resorcinol diphosphate; hydroquinone diphosphate; bisphenol-A polyphosphate; and the like.
The phosphorous-based compounds are often very effective for increasing the flame retardancy of PPE resins, as well as for resins based on PPE and high impact (rubber-modified) polystyrene (HIPS). However, as described in the Patel patent, the presence of phosphates and diphosphates—especially in large concentrations—can lower the heat deflection temperature (HDT) of the final product. A significant decrease in HDT characteristics is sometimes very unacceptable for a variety of end uses.
In addition to flame retardancy, PPE resins often need to exhibit a particular set of smoke characteristics under burning conditions. These characteristics are often critical when the compositions are used for products like wire insulation and ceiling tiles. The styrene content in many PPE formulations can undesirably increase smoke density. Moreover, the presence of large amounts of organic phosphate flame retardants (which may also function as aromatic plasticizers) can exacerbate smoke conditions in many situations.
It should thus be apparent that a need continues to exist for PPE compositions (with or without rubber-modified polystyrene or other polymers), which exhibit very good flame-retardant characteristics. Compositions of this type which do not rely on the use of large amounts of halogen-based flame retardants would also be welcome in the art. Furthermore, it would be particularly advantageous if flame-retardancy were achieved without a large decrease in high-heat properties, like HDT. Moreover, the compositions should also exhibit one or more desirable smoke characteristics, e.g., relatively low smoke density. It would often be advantageous if PPE compositions meeting this criteria were to also substantially retain other important characteristics as well, such as impact strength and hydrolytic stability.